Tire with silica reinforced tread

Abstract: The invention relates to a tire with a tread which is reinforced with silica where the tread is composed of at least three rubbers selected from isoprene/butadiene copolymer rubber, 3,4-polyisoprene rubber, cis 1,4-polybutadiene rubber and which may also contain cis 1,4-polyisoprene natural rubber.

Description: Field This invention relates to a tire having a rubber tread which is reinforced with silica. In one aspect, the tread is comprised of a specified multiple component rubber blend reinforced with a quantitative amount of silica or a combination of silica and carbon black. Background Pneumatic rubber tires are conventionally prepared with a rubber tread which can be a blend of various rubbers which is typically reinforced with carbon black. In one aspect, rubbers are evaluated, selected and blended for a purpose of achieving desired tire tread properties and particularly a balance of tire tread characteristic properties, mainly, rolling resistance, traction and wear. For various applications utilizing rubber including applications such as tires and particularly tire treads, sulfur cured rubber is utilized which contains substantial amounts of reinforcing filler(s). Carbon black is commonly used for such purpose and normally provides or enhances good physical properties for the sulfur cured rubber. Particulate silica is also sometimes used for such purpose, particularly when the silica is used in conjunction with a coupling agent. In some cases, a combination of silica and carbon black is utilized for reinforcing fillers for various rubber products, including treads for tires. It is important to appreciate that, conventionally, carbon black is considered to be a more effective reinforcing filler for rubber tire treads than silica if the silica is used without a coupling agent. Indeed, at least as compared to carbon black, there tends to be a lack of, or at least an insufficient degree of, physical and/or chemical bonding between the silica particles and the rubber elastomers to enable the silica to become a reinforcing filler for the rubber for most purposes, including tire treads, if the silica is used without a coupler. While various treatments and procedures have been devised to overcome such deficiencies, compounds capable of reacting with both the silica surface and the rubber elastomer molecule, generally known to those skilled in such art as coupling agents, or couplers, are often used. Such coupling agents, for example, may be premixed, or pre-reacted, with the silica particles or added to the rubber mix during the rubber/silica processing, or mixing, stage. If the coupling agent and silica are added separately to the rubber mix during the rubber/silica mixing, or processing stage, it is considered that the coupling agent then combines in situ with the silica. In particular, such coupling agents are generally composed of a silane which has a constituent component, or moiety, (the silane portion) capable of reacting with the silica surface and, also, a constituent component, or moiety, capable of reacting with the rubber, particularly a sulfur vulcanizable rubber which contains carbon-to-carbon double bonds, or unsaturation. In this manner, then the coupler acts as a connecting bridge between the silica and the rubber and thereby enhances the rubber reinforcement aspect of the silica. In one aspect, the silane of the coupling agent apparently forms a bond to the silica surface, possibly through hydrolysis, and the rubber reactive component of the coupling agent combines with the rubber itself. Usually the rubber reactive component of the coupler is temperature sensitive and tends to combine with the rubber during the final and higher temperature sulfur vulcanization stage and, thus, subsequent to the rubber/silica/coupler mixing stage and, therefore, after the silane group of the coupler has combined with the silica. However, partly because of typical temperature sensitivity of the coupler, some degree of combination, or bonding, may occur between the rubber-reactive component of the coupler and the rubber during an initial rubber/silica/coupler mixing stages and, thus, prior to a subsequent vulcanization stage. The rubber-reactive group component of the coupler may be, for example, one or more of groups such as mercapto, amino, vinyl, epoxy, and sulfur groups, preferably a sulfur or mercapto moiety and more preferably sulfur. Numerous coupling agents are taught for use in combining silica and rubber, such as for example, silane coupling agents containing a polysulfide component, or structure, such as bis-(3-triethoxysilylpropyl) tetrasulfide (e.g. U.S. Patent No. 3,873,489). For silica reinforced tire treads, U.S. Patent No. 5,066,721, in its Comparative Test Example 1 in Table 3 (column 15), discloses the use of solution polymerization prepared SBR containing 50 parts silica for a tire tread. Table 4 (column 17) illustrates the tire preparation. U.S. Patent No. 5,227,425 discloses the use of a solution polymerization prepared SBR which is silica reinforced with a specified silica characterization and in which is preferenced over an emulsion polymerization prepared SBR. U.S. Patent No. 4,519,430 discloses a silica rich tire tread which contains solution or emulsion SBR, optionally with polybutadiene rubber and/or polyisoprene rubber together with a mixture of silica and carbon black, with silica being required to be a major component of the silica/carbon black reinforcing filler. Other U.S. patents relating to silicas and silica reinforced tire treads include U.S. Patents Nos. 3,451,458; 3,664,403; 3,768,537; 3,873,489; 3,884,285; 3,938,574; 4,482,663; 4,590,052, 5 087 668, 5,089,554 and British 1,424,503. Summary and Practice of the Invention In accordance with this invention, a pneumatic tire is provided having a sulfur vulcanized tread comprised of, based on 100 parts by weight rubber, (A) at least three diene-based elastomers comprised of (i) 15 to 50, preferably 15 to 40, phr of isoprene/butadiene copolymer rubber containing 40 to 60 percent isoprene and having a Tg of -40 to -50°C, (ii) 10 to 40, preferably 10 to 30, phr of 3,4-polyisoprene having a Tg in a range of -15°C to -25°C, (iii) 10 to 30, preferably 15 to 30, phr of a cis 1,4-polybutadiene rubber, and (iv) 0 to 30, preferably 10 to 20, phr of cis 1,4-polyisoprene natural rubber, (B) 50 to 110, preferably 60 to 85, phr particulate silica, (C) at least one silica coupler having a silane moiety reactive with silicon dioxide and a sulfur moiety reactive with said elastomer, in a weight ratio of silica to coupler of 7/1 to 15/1, and (D) 0 to 50 phr carbon black, wherein the weight ratio of silica to carbon black, if carbon black is used, is at least 2/1, preferably at least 4/1 and more preferably at least 10/1 and where the total of silica and carbon black, if used, is 60 to 120, preferably 70 to 90 phr. In a preferred embodiment of the invention, a pneumatic tire is provided having a sulfur vulcanized tread comprised of, based on 100 parts by weight rubber, (A) diene-based elastomers comprised of (i) 15 to 40 phr of isoprene/butadiene copolymer rubber containing 40 to 60 percent isoprene and having a Tg in a range of -40 to -50°C, (ii) 10 to 30 phr of a 3,4-polyisoprene rubber having a Tg in a range of -15°C to -25°C, (iii) 15 to 30 phr of a cis 1,4-polybutadiene rubber, and 10 to 20 phr of cis 1,4-polyisoprene natural rubber, (B) 50 to 85, preferably 60 to 85, phr particulate silica, (C) at least one silica coupler having a silane moiety reactive with silicon dioxide and a sulfur moiety reactive with said elastomer, in a weight ratio of silica to coupler of 7/1 to 15/1, and (D) 0 to 50 phr carbon black, wherein the weight ratio of silica to carbon black, if carbon black is used, is at least 2/1, preferably at least 4/1 and more preferably at least 10/1 and for some applications at least 10/1 and where the total of silica and carbon black, if carbon black is used, is 60 to 120, preferably 70 to 90 phr. In a further preferred embodiment of the invention, a pneumatic tire is provided having a sulfur vulcanized tread comprised of, based on 100 parts by weight rubber, (A) diene-based elastomers comprised of (i) 40 to 60 phr of emulsion polymerization prepared isoprene/butadiene copolymer rubber containing 40 to 60 percent isoprene and having a Tg in a range of -40 to -50°C, (ii) 10 to 30 phr of a 3,4-polyisoprene rubber having a Tg in a range of -15°C to -25°C, and (iii) 10 to 30 phr of cis 1,4-polyisoprene natural rubber, (B) 50 to 85, preferably 60 to 85, phr particulate silica, (C) at least one silica coupler having a silane moiety reactive with silicon dioxide and a sulfur moiety reactive with said elastomer, in a weight ratio of silica to coupler of 7/1 to 15/1, and (D) 0 to 50 phr carbon black, wherein the weight ratio of silica to carbon black, if carbon black is used, is at least 2/1, preferably at least 4/1 and for some applications at least 10/1 and where the total of silica and carbon black, if carbon black is used, is 60 to 120, preferably 70 to 90 phr. The term "phr" as used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber". The rubber blends containing at least three rubbers is an important feature of the invention designed to enhance properties of a tire tread containing a substantial amount of silica reinforcement. In one aspect, the isoprene/butadiene copolymer rubber (IBR) is required to have an isoprene content of 40 to 60 percent and a Tg in a range of -40 to -50°C. The 3,4-polyisoprene rubber (3,4-PI) is considered beneficial to enhance the tire's traction. The 3,4-PI and use thereof is more fully described in U.S. Patent No. 5,087,668. The Tg refers to the glass transition temperature which can conveniently be determined by a differential scanning calorimeter at a heating rate of 10°C per minute. The cis 1,4-polybutadiene rubber (BR) is considered to be beneficial to enhance the tire tread's wear, or treadwear. Such BR can be prepared, for example, by organic solution polymerization of 1,3-butadiene. The BR may be conventionally characterized, for example, by having at least a 90% cis 1,4-content. The cis 1,4-polyisoprene natural rubber is well known to those having skill in the rubber art. Thus, in the practice of this invention, a balanced rubber blend of at least three rubbers is provided which relies upon silica reinforcement which, in turn, relies on a silica coupler for the silica's reinforcing effect for the rubber blend. In another aspect, when such sulfur vulcanized rubber tread also may contain carbon black a weight ratio of silica to carbon black is at least 2/1, preferably at least 4/1 and for some applications at least 10/1. The commonly employed siliceous pigments used in rubber compounding applications can be used as the silica in this invention, including pyrogenic and precipitated siliceous pigments (silica), although precipitate silicas are preferred. The siliceous pigments preferably employed in this invention are precipitated silicas such as, for example, those obtained by the acidification of a soluble silicate, e.g., sodium silicate. The siliceous pigment (silica) should, for example, have an ultimate particle size in a range of 50 to 10,000 angstroms, preferably between 50 and 400 angstroms. The BET surface area of the pigment, as measured using nitrogen gas, is preferably in the range of 100 to 250, preferably 120 to 180, square meters per gram. The BET method of measuring surface area is described in the Journal of the American Chemical Society , Volume 60, page 304 (1930). The silica also typically has a dibutylphthalate (DBP) absorption value in a range of 200 to 400, and usually 220 to 300. The silica might be expected to have an average ultimate particle size, for example, in a range of 0.01 to 0.05 micron as determined by the electron microscope, although the silica particles may be even smaller in size. While the "projected" area of silicas, before and after mixing with rubber, has been sometimes taught to be suitable to characterize various silicas, it is considered that such characterizations are insufficient, or unreliable unless adequate sample preparation is designated and defined because the electron microscope determination of projected area of the silica is considered to be largely dependent upon sample preparation. Preparation variables include sample container size and mixing energy and need to be clarified in complete detail. Various commercially available silicas may be considered for use in this invention such as, for example only and without limitation, silicas commercially available from PPG Industries under the Hi-Sil trademark with designations 210, 243, etc; silicas available from Rhone-Poulenc, with designations of Z1165MP and Z165GR and silicas available from Degussa AG with designations VN2 and VN3, etc. The PPG Hi-Sil silicas are preferred. It is readily understood by those having skill in the art that the rubber composition of the tread rubber would be compounded by methods generally known in the rubber compounding art, such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly used additive materials such as, for example, curing aids, such as sulfur, activators, retarders and accelerators, processing additives, such as oils, resins including tackifying resins, silicas, and plasticizers, fillers, pigments, fatty acid, zinc oxide, waxes, antioxidants and antiozonants, peptizing agents and reinforcing materials such as, for example, carbon black. As known to those skilled in the art, depending on the intended use of the sulfur vulcanizable and sulfur vulcanized materials (rubbers), the additives mentioned above are selected and commonly used in conventional amounts. Typically additions of carbon black, for this invention, if used, are hereinbefore set forth. Typical amounts of tackifier resins, if used, comprise 0.5 to 10 phr, usually 1 to 5 phr. Typical amounts of processing aids comprise 1 to 50 phr. Such processing aids can include, for example, aromatic, napthenic, and/or paraffinic processing oils. Typical amounts of antioxidants comprise 1 to 5 phr. Representative antioxidants may be, for example, diphenyl-p-phenylenediamine and others, such as, for example, those disclosed in the Vanderbilt Rubber Handbook (1978), pages 344-346. Typical amounts of antiozonants comprise 1 to 5 phr. Typical amounts of fatty acids, if used, which can include stearic acid comprise 0.5 to 3 phr. Typical amounts of zinc oxide comprise 2 to 5 phr. Typical amounts of waxes comprise 1 to 5 phr. Often microcrystalline waxes are used. Typical amounts of peptizers comprise 0.1 to 1 phr. Typical peptizers may be, for example, pentachlorothiophenol and dibenzamidodiphenyl disulfide. The vulcanization is conducted in the presence of a sulfur vulcanizing agent. Examples of suitable sulfur vulcanizing agents include elemental sulfur (free sulfur) or sulfur donating vulcanizing agents, for example, an amine disulfide, polymeric polysulfide or sulfur olefin adducts. Preferably, the sulfur vulcanizing agent is elemental sulfur. As known to those skilled in the art, sulfur vulcanizing agents are used in an amount ranging from 0.5 to 4 phr, or even, in some circumstances, up to 8 phr, with a range of from 1.5 to 2.5, sometimes from 2 to 2.5, being preferred. Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. Retarders are also used to control the rate of vulcanization. In one embodiment, a single accelerator system may be used, i.e., primary accelerator. Conventionally and preferably, a primary accelerator(s) is used in total amounts ranging from 0.5 to 4, preferably 0.8 to 1.5, phr. In another embodiment, combinations of a primary and a secondary accelerator might be used, with the secondary accelerator being used in amounts of 0.05 to 3 phr, for example, in order to activate and to improve the properties of the vulcanizate. Combinations of these accelerators might be expected to produce a synergistic effect on the final properties and are somewhat better than those produced by use of either accelerator alone. In addition, delayed action accelerators may be used which are not affected by normal processing temperatures but produce a satisfactory cure at ordinary vulcanization temperatures. Suitable types of accelerators that may be used in the present invention are amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates. Preferably, the primary accelerator is a sulfenamide. If a second accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate or thiuram compound. The presence and relative amounts of sulfur vulcanizing agent and accelerator(s) are not considered to be an aspect of this invention which is more primarily directed to the use of silica as a reinforcing filler in combination with a coupling agent. The presence and relative amounts of the above additives are not considered to be an aspect of the present invention which is more primarily directed to the utilization of specified blends of rubbers in tire treads, in combination with silica and silica coupler. The tire can be built, shaped, molded and cured by various methods which will be readily apparent to those having skill in such art. The invention may be better understood by reference to the following examples in which the parts and percentages are by weight unless otherwise indicated. EXAMPLE I Rubber compositions (compounded rubber) were prepared of various blends of three of isoprene/butadiene copolymer rubber (IBR), 3,4-polyisoprene rubber (3,4-PI), cis 1,4-polybutadiene rubber (BR) and optionally cis 1,4-polyisoprene natural rubber (NR) and referred to herein as Sample X and Sample Y. A control rubber composition composed of solution polymerization prepared styrene/butadiene copolymer rubber (S-SBR) and cis 1,4-polybutadiene rubber was prepared and identified herein as Sample Z. The rubber compositions were prepared by mixing the ingredients in several stages, namely, two non-productive stages (without the curatives) and a productive stage (basically for the curatives), then the resulting composition was cured under conditions of elevated temperature and pressure. For the non-productive mixing stages, exclusive of the accelerator(s), sulfur curatives zinc oxide and antioxidant which are mixed (added) in the final, productive mixing stage, all of the ingredients were mixed in the first non-productive stage except for 20 to 50 percent of the reinforcing fillers (silica or carbon black), with proportional amounts (to the reinforcing fillers) of coupler and processing oil, which were added in the second non-productive mixing stage. The ingredients were mixed in each of the non-productive mixing stages for 5 minutes to a temperature of 165°C, all in a Banbury type of mixer. To the resulting rubber composition (mixture) was then mixed, in the productive mixing stage, the remaining ingredients in a Banbury type mixer for 2 minutes to a temperature of 110°C. The rubber was then vulcanized at a temperature of 150°C for 18 minutes. The rubber composition was comprised of the ingredients illustrated in Table 1. Table 2 illustrates properties of the cured rubber composition. Sample # X (C) Y (I) Z (C) Rheometer (150°C) Max. Torque, dNm 21.2 20.7 19.4 Min. Torque, dNm 5.5 5.4 2.7 T 90 , minutes 14.4 14.7 18.0 T 25 , minutes 10.0 10.1 10.3 Stress-Strain Tensile Strength, MPa 13.4 13.4 12.5 Elongation at Break, % 442 459 464 100% Modulus, MPa 2.7 2.7 2.6 300% Modulus, MPA 9.4 9.1 8.3 Rebound 100°C, (%) 65 64 63 23°C (%) 45 40 47 Hardness (Shore A) 23°C 62 62 64 100°C 58 59 61 DIN Abrasion (Relative Volume Loss) cm 3 87 84 131 Dynamic Mechanical Properties E* at 60°C, MPa 8.63 11.1 13.7 E* at 0°C, MPa 20.7 26.2 30.0 Tan Delta at 60°C 0.081 0.093 0.113 Tan Delta at 0°C 0.187 0.232 0.241 These properties of the rubber compositions demonstrate an excellent balance of rolling resistance, traction and treadwear. Sample X demonstrated slightly lower tan delta at 60°C as compared to Sample Y which is indicative of improved tire rolling resistance when the composition is used as a tire tread whereas Sample Y demonstrated a slightly higher tan delta at 0°C as compared to Sample X which is indicative of improved traction when the composition is used as a tire tread which is to be a contribution of the 3,4-polyisoprene. Both of Sample X and Y demonstrated superior abrasion resistance and a lower tan delta at 60°C as compared to Sample Z which is indicative of better treadwear and reduced rolling resistance for a tire with a tread of such composition.

Claims: A pneumatic tire having a sulfur vulcanized tread characterized by being comprised of, based on 100 parts by weight rubber, (A) at least three diene-based elastomers comprised of (i) 15 to 50 phr of isoprene/butadiene copolymer rubber containing 40 to 60 percent isoprene and having a Tg in a range of -40°C to -50°C, (ii) 10 to 40 phr of a 3,4-polyisoprene rubber having a Tg in a range of -15°C to -25°C, (iii) 10 to 30 phr of a cis 1,4-polybutadiene rubber, and (iv) 0 to 30 phr of cis 1,4-polyisoprene natural rubber, (B) 50 to 110 phr particulate silica, (C) at least one silica coupler having a silane moiety reactive with silicon dioxide and a sulfur moiety reactive with said elastomer, in a weight ratio of silica to coupler of 7/1 to 15/1, and (D) 0 to 50 phr carbon black, wherein the weight ratio of silica to carbon black, if carbon black is used, is at least 2/1 where the total of silica and carbon black, if carbon black is used, is 60 to 120 phr. The tire of claim 1 characterized in that the tread contains both silica and carbon black and the weight ratio of silica to carbon black is at least 2/1. The tire of claim 1 being comprised of, based on 100 parts by weight rubber, (A) diene-based elastomers comprised of (i) 15 to 40 phr of isoprene/butadiene copolymer rubber containing 40 to 60 percent isoprene and having a Tg in a range of -40 to -50°C, (ii) 10 to 30 phr of a 3,4-polyisoprene rubber having a Tg in a range of -15°C to -25°C, (iii) 15 to 30 phr of a cis 1,4-polybutadiene rubber, and (iv) 10 to 20 phr of cis 1,4-polyisoprene natural rubber, (B) 60 to 110 phr particulate silica, (C) at least one silica coupler having a silane moiety reactive with silicon dioxide and a sulfur moiety reactive with said elastomer in a weight ratio of silica to coupler of 7/1 to 15/1, and (D) 0 to 50 phr carbon black, wherein the weight ratio of silica to carbon black, if carbon black is used, is at least 4/1 where the total of silica and carbon black, if carbon black is used, is 70 to 120 phr. The tire of claim 1 or 3 characterized in that said silica is characterized by having a BET surface area in a range of 100 to 250 and a DBP absorption value in a range of 200 to 400. The tire of claim 1 or 3 characterized in that the said coupler is a bis-3-(triethoxysilylpropyl)tetrasulfide. The tire of claim 1 or 3 characterized in that the tread contains both silica and carbon black and the weight ratio of silica to carbon black is at least 4/1. The tire of claim 1 or 2 characterized in that the tread contains both silica and carbon black and the weight ratio of silica to carbon black is at least 10/1.

Inventors: CRAWFORD MICHAEL JULIAN, LOSEY CHERYL ANN, SANDSTROM PAUL HARRY, VERTHE JOHN JOSEPH ANDRE, ZANZIG DAVID JOHN, CRAWFORD, MICHAEL JULIAN, LOSEY, CHERYL ANN, SANDSTROM, PAUL HARRY, VERTHE, JOHN JOSEPH ANDRE, ZANZIG, DAVID JOHN
Applicants: GOODYEAR TIRE & RUBBER, THE GOODYEAR TIRE & RUBBER COMPANY
Application year: 1994
Main classification: C08L9
IPC codes: C08K3, C08L21, C08L9, B60C1